1. Field of the Invention
This invention relates to an active filter adaptable to be utilized either as a notch filter or as a band pass filter.
2. Description of the Prior Art
Active filters were first introduced in the thirties, but they provided relatively little competition vis-a-vis the passive LC filters then in use. The competitive advantage resided with the passive filters by reason of the fact that the active filters were required to use vacuum tubes as gain elements which resulted in a configuration which was both cumbersome as well as short lived.
All this was changed with the introduction of the transistor and the accelerated development which followed in solid state electronics both as regards reliability as well as miniaturzation.
One of the best discussions of active filters can be found in the treatise Active Filter Design by Arthur B. Williams published by Artech House Inc. 610 Washington St., Dedham, Mass. 02026, Copyright 1975. Another excellent discussion of active filters may be found in Operational Amplifiers by Jerald G. Graeme, Gene E. Tobey and Lawrence P. Huelsman published by McGraw-Hill copyright 1971 (See pages 282-326).
The difficulty with prior art active filters has been the problem of adjusting commercially available components to achieve a desired frequency response. The problem is stated in Active Filter Design by Arthur B. Williams at page 3:
"A major disadvantage of Active Filters has been the lack of adjustment capability, thus placing the Frequency Response at the mercy of component tolerances whereas passive filters have tunable Inductors".